The mechanisms responsible for inducing and maintaining tolerance will be examined in the mouse model of Total Lymphoid Irradiation (TLI). TLI is currently successfully used to prepare patients for bone marrow and kidney transplantation. Because TLI treated transplant patients exhibit higher incidences of donor specific tolerance than conventionally treated patients, understanding how TLI triggers cell unresponsiveness or tolerance may help develop more effective less toxic transplant protocols. Current theory proposes that natural suppressor cells which arise in TLI treated mice induce tolerance, but do not maintain tolerance. However, the roles of anergy and deletion in either the induction or the maintenance phase of TLI tolerance have not been fully examined. Aims I-III deal with mechanisms of tolerance induction and Aims IV and V with tolerance maintenance. Specific aim I investigates whether cells from TLI treated mice trigger normal T cells into a state of anergy, using two in vitro systems, an allogeneic MLR and an antigen specific T hybridoma. Aim II determines (i) whether TLI cells block or alter the ability of T cells to transcribe IL-2 RNA using RNA in situ hybridization and (ii) whether TLI cells induce changes in the high affinity TL-2 receptor expressed on T cells using standard scatchard analysis. Aim III evaluates if TLI cells alter expression of other cytokines, such as IFN-gamma, IL-10 and IL-4 by examining whether TLI cells similarly effect Th1 and Th2 helper T cell clones. Aim IV examines whether TLI maintains tolerance through mechanisms of suppression, deletion or anergy using TLI prepared allogeneic bone marrow chimeras as an in vivo tolerance model. Finally, aim V examines whether unresponsive T cells from the in vivo model exhibit similar changes in cytokine expression as in vitro TLI treated cells. These studies are clinically significant because they examine tolerance mechanisms in a model which utilizes a clinically proven therapy, TLI. Second, the studies help define whether similar steps in the IL-2 pathway are blocked during TLI tolerance induction and maintenance and help clarify how TLI tolerance differs from other tolerance systems. Third, the studies establish whether tolerance involves blocking other cytokines besides IL-2. The results may provide rationale for combining different immunosuppressive therapies in clinical settings. Finally, correlating the in vitro and in vivo findings validates using an in vitro model for future studies that examine which activation steps TLI cells block.